Method of and apparatus for continuously foaming a polyimide powder

ABSTRACT

A method of and apparatus for continuously foaming a polyimide prepolymer powder which is not susceptible to heating by microwave energy utilize a preheated conveyor belt to heat the powder in contact with and adjacent the surface of the conveyor belt to commence the reaction and foaming of the powder as it enters an oven which further heats the powder by convection and infrared radiation to form a continuous foam bun. The conveyor belt is cooled as it exits the oven to cool a bottom surface of the foam product to enable the foam product to be removed from the conveyor before interior portions of the foam product have cooled. Foam adhering to the conveyor belt, after the product has been removed, is cleaned from the conveyor belt, collected and mixed with and used as a filler in the prepolymer powder used in the process. In addition the mass of the powder entering the oven and the density and degree of cure of the foam bun exiting the oven are monitored to continuously control the process.

BACKGROUND OF THE INVENTION

The invention relates to polyimide foams and in particular to a methodof and apparatus for continuously foaming a polyimide prepolymer powderthat is not susceptible to microwave heating wherein the polyimideprepolymer powder is uniformly heated throughout to form a continuouspiece of foam product of relatively uniform cell size, density andstrength throughout.

Many foams and especially polyimide foams are produced by batchprocesses where the prepolymer powder is foamed in a closed or open moldor by the free rise method. As discussed in U.S. Pat. No. 4,780,167,(columns 1 and 2), the foam produced in closed molds tends to be veryirregular and lack uniformity of cell size, density and strength. Theproblems increase when attempts are made to vary the foam productdensity by varying the amount of precursor placed in the mold. Whilefoaming product under ambient pressure in a free rise process or in openmolds produces more uniform cell size and density, it is still difficultto produce foam panels of controlled, varying densities for differentapplications. In addition, when using batch operations to foampolyimides, it is necessary to use masking materials or releasematerials to prevent the foam from adhering to the mold contact surfacesand/or the mold contact surfaces must be cleaned between batches. In afree rise batch process as discussed in U.S. Pat. No. 4,305,796, (column5, line 51), the faces and all four sides of the buns produced in thebatch process normally must be trimmed to provide the end product withsmooth planer surfaces. This results in trim or scrap losses whichincrease the cost of the end product.

The '167 patent further discusses the foaming of certain resins betweenmoving endless belts. The patent states that while this method offoaming is apparently effective with polystyrene and polyurethane "thismethod is not applicable to controlled density polyimide foam because ofthe much longer periods at much higher temperatures required forpolyimides which require excessively long and/or very slowly movingbelts operating in a very high temperature oven."

In addition, the heat transfer situation is very different for apolyimide foam when compared to phenolic and polyurethane foams whichare formed from ingredients that are much more reactive than theingredients used to produce polyimide foam. With the phenolic andpolyurethane foams, the reactive ingredients are mixed together in amixing head and are deposited within seconds onto the conveyor where theingredients react and produce heat. The blowing agent is a volatilecompound (such as a chlorofluorocarbon) that has been added to themixture specifically for blowing. In this process, the oven's heat isused to raise and/or keep the temperatures of the outer surfaces of thefoam at the same temperatures as those generated within the foam duringthe foaming process due to the exotherm of the reacting ingredients.

With polyimide foam, the ingredients react more slowly and the heat fromthe oven is required to promote the reaction of the ingredientsthroughout the foam. As the ingredients in the upper portion of thepolyimide powder are foamed, the upper foamed portion insulates thelower portions of the ingredients from the hot oven air or infraredradiation thereby retarding the foaming process and producing nonuniformtemperatures throughout the thickness of the ingredients being foamed.

U.S. Pat. No. 4,900,761, is directed to a process for producing apolyimide foam wherein "In at least one such stage the precursor issubjected to one or more temperatures sufficient to obtain aconsolidated but friable cellular foam structure, and in at least oneother such stage this cellular foam structure is subjected to one ormore higher temperatures sufficient to cure the cellular material into aresilient polyimide foam. Preferably, these stages are conducted in acontinuous manner as by supporting the material being foamed on a movingbelt or rotating platform associated with appropriate heatingapparatus...". While directed to a continuous process of foaming, theprocess differs in its approach from the method of the present inventionin certain important respects and does not address the need for uniformheating of the ingredients throughout the entire thickness of thematerials being foamed by balancing the heat imparted to the foam fromabove and below by preheating the moving belt. In addition, it does notaddress the need to rapidly cool the lower surface of the foam productformed so that the foam can be removed from the moving belt before theinterior of the foam is completely cool to increase the efficiency ofthe continuous foaming process.

U.S. Pat. No. 4,855,331, is directed to the production of foamed polymerstructures in a batch process which uses microwave radiation to foamprecursors that are susceptible to microwave heating. In this batchprocess microwave radiation is applied to the precursor from above, thesides and the ends of the precursor body. However a shield is placedbelow the precursor body to curtail radiation directed at the precursorbody from below. The shield can be heated to a temperature of betweenabout 50 degrees and about 200 degrees Centigrade (preferably in therange of about 60 degrees to about 130 degrees Centigrade) to raise thetemperature of at least the lower portion of the precursor body.

U.S. Pat. No. 4,305,796, is directed to a method of preparing polyimidesfrom precursors which are susceptible to microwave heating in a batchprocess using microwave radiation. In this batch process the substrateor mold can be preheated to a temperature of about 121 degreesCentigrade to 149 degrees Centigrade.

SUMMARY OF THE INVENTION

The present invention provides a method of and apparatus forcontinuously foaming a polyimide prepolymer powder of the type that isnot susceptible to microwave heating under ambient pressure to produce apolyimide foam product which has more uniform cell size, density andstrength throughout. A preheated conveyor belt is utilized to convey alayer of polyimide prepolymer powder through a conventional convectionand radiant heat curing oven. Due to the nature of the polyimideprepolymer powder, microwave heating is not utilized in the process.Heat conduction from the conveyor belt into the polyimide prepolymerpowder is the main heat source for the polyimide prepolymer powdercomprising the lower portion (approximately the lower one half) of thepowder layer and especially to the powder contacting and adjacent theupper surface of the conveyor belt. The main heat sources for the upperhalf of the powder layer are from oven air convection and oven surfaceinfrared radiation.

If the prepolymer powder were spread onto a cold conveyor belt andconveyed into an oven, the upper surface of the prepolymer powder layerwould take on heat by convection and radiation and the lower surface ofthe prepolymer powder layer would be kept relatively cool due to heatconduction by the conveyor belt. The upper portion of the prepolymerpowder layer would begin to foam as it is heated by the oven and beginto develop a cellular structure thereby insulating the lower portion ofthe prepolymer powder layer limiting or retarding the absorption of heatby the lower portion of the prepolymer powder from above. The heating ofthe lower portion of the prepolymer powder layer would take place frombelow receiving its heat by conduction from the conveyor belt- However,before the conveyor belt would provide heat to the lower portion of theprepolymer powder, the conveyor belt would have to be heated. Thus,initially the conveyor belt would act as a heat sink retarding theheating of the lower portion of the powder layer. This would affect thecomparative cell size, density and strength of the upper and the lowerportions of the finished foam product. Since the upper portion of thepowder layer would become heated before the lower portion of the powderlayer, at some point in the foaming process, the upper portion would beessentially in a stabilized foamed condition, having a developedcellular structure, while the lower portion of the powder layer wouldstill be foaming and developing its cellular structure. This wouldrestrain formation of cells in the lower portion of the powder layer andwould cause defects such as, cell collapse and nonuniform cell size. Inaddition, the upper portion of the foamed product could overheat, turn adarker color and possibly, experience property degradation before thefoaming of the lower portion of the powder layer was completed. Thus,the use of the preheated conveyor belt in the continuous process of thepresent invention greatly improves the overall appearance, performanceand quality of the finished product.

In addition to improving the quality of the foamed product produced, thepreheating of the conveyor belt speeds up the heat transfer to theprepolymer powder thereby decreasing the time required for foaming thepowder. This permits an increase in production line speeds and thus anincrease in product output. It is estimated that the use of the heatedconveyor belt could increase production speeds by from 50% to 200%depending on the thickness of the product being produced.

The conveyor belt used in the present invention has a high thermalconductivity and a relatively low heat capacity. This permits theconveyor belt to be rapidly preheated and facilitates a rapid heattransfer to the prepolymer powder layer. It also permits the conveyorbelt to be rapidly cooled upon exiting the oven to cool the lowersurface of the continuous foam bun so that the foam product can beremoved from the conveyor belt sooner.

When it is reacted, polyimide precursor powder has good adhesiveproperties. In batch operations, this necessitates the use of maskingmaterials, release materials and/or the cleaning of the mold contactsurfaces between foaming cycles. However, with the continuous processand apparatus of the present invention, after the product is removedfrom the conveyor belt, the conveyor belt is continuously andautomatically cleaned thereby eliminating this labor intensive step ofthe prior art batch processes. The polyimide cleaned or removed from theupper surface of the conveyor belt can be ground and added to theprepolymer powder being spread on the conveyor belt as it enters theoven, serving as a filler material to further reduce the raw materialand production costs.

The process and apparatus of the present invention also include theability to post cure and/or crush the foam bun produced as part of thecontinuous process for manufacturing the foam bun. Since the foam bunproduced is continuous, the process and apparatus of the presentinvention eliminate end trim loss that is present in batch operations.These features of the process and apparatus of the present inventionreduce the handling of the bun by the process operators and provide amore efficient and cost effective operation than the prior art batchprocesses.

The continuous process of the present invention is to be controlled bybeta gages and/or dielectric meters. However, even if these controlmeans are not used, the continuous process of the present inventionstill offers control advantages over the batch processes of the priorart. In actual practice, an operator would be better able to make fineadjustments to the process based on a continuous visual assessment ofthe foam product as the foam product exits the oven or as the productcomes off the end of the production line than he could based on anassessment of buns made in a batch process. In a batch process, theprepolymmer powder is segregated into batches, the powder for each bunis laid out in a discrete operation, the oven doors are opened, the ovenis charged and the oven is closed. Each of these steps can introduce avariation peculiar to the specific batch. The continuous process of thepresent invention eliminates many of these potential variables.

The polyimide foam produced by the method and apparatus of the presentinvention is one that has increased fire retardancy compared topolyurethane foam. The product may be used in confined areas e. g-,aircraft cabins, below decks in ships and in similar applications. Inaddition, the foam product can be adhered to selected substrates, usedbetween walls or adhesively bonded to the surface of a wall.

A preferred polyimide foam which is not susceptible to microwave heatingand which can be produced by the method and apparatus of the presentinvention has a ratio of imide to amide groups in the final product thatis greater than a 1 to 1 ratio, ranging from approximately 1 toapproximately 19 imide groups per amide group (as used herein "polyimidefoam"). It is a light weight open cell foam having a density of lessthan 6 pounds per cubic foot and preferably less than 1.0 pounds percubic foot and greater than 0.20 pounds per cubic foot. It ischaracterized by the following physical properties: high thermalstability, low shrinkage when exposed to high service temperatures, andlow density.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic elevation of the apparatus of the presentinvention illustrating its use in the method of continuously foamingpolyimide prepolymer powder under ambient pressure to produce apolyimide foam product.

FIG. 2 is a cross-sectional view taken substantially along line 2--2 ofFIG. 1 to show a beta gage for monitoring the mass of the prepolymerpowder layer.

FIG. 3 is a cross-sectional view taken substantially along lines 3--3 ofFIG. 1 to show a beta gage for monitoring the density of the foamproduct exiting the oven.

FIG. 4 is a cross-sectional view also taken substantially along lines4--4 of FIG. 1 showing a dielectric meter being used to monitor thedegree of cure of the foam product exiting the oven.

FIG. 5 is a partial elevation view of the upstream end of the productionline showing an alternative way of preheating the conveyor belt.

DETAILED DESCRIPTION OF THE INVENTION

While the method and apparatus of the present invention are generallyapplicable to heating a prepolymer powder at ambient pressure to releasea blowing agent and continuously form a foamed product, the method andapparatus are especially designed to continuously form a foamed productfrom a polyimide prepolymer powder which is not susceptible to microwaveheating. The method and apparatus of the present invention preferablyemploy a two stage reaction as described in U.S. Pat. No. 4,980,387,issued Dec. 25, 1990, and entitled Lightweight Flexible Imide Foam andMethod of Producing Same and U.S. Pat. No. 4,990,543, issued Feb. 5,1991, and entitled Lightweight Flexible Foam and Method of Producing theSame, both of which are incorporated herein in their entirety byreference.

The polyimide foam produced in accordance with the method and apparatusof the present invention is generally produced in a two or three stepprocess. A polyisocyanate having at least two functional groups permolecule and a mixture of a difunctional acid and a difunctionalanhydride in the presence of a tertiary amine hydrazine catalyst can beused for the reactants. The reactants are blended together and reactedin the absence of a solvent by being heated to approximately 140 to 230degrees Centigrade. In one or two steps, approximately 10% to 80% andpreferably 45% to 75% of the theoretical carbon dioxide available fromthe ingredients is generated. The reaction mixture is cooled, and groundinto a fine prepolymer powder. The prepolymer powder material is furtherreacted to completion at temperatures of from approximately 200 to 290degrees Centigrade in a continuous foaming process where the carbondioxide generation is completed.

As discussed above, in the polyimide foam product produced by the methodand apparatus of the present invention, the ratio of the imide groups toamide groups is greater than a 1 to 1 ratio, ranging from approximately1 to approximately 19 imide groups per amide group. The finished producthas a density of less than 6 pounds per cubic foot and preferablybetween 0.20 and 1.0 pounds per cubic foot.

Referring now to the FIG. 1, an apparatus 10 is shown for carrying outthe method of the present invention wherein a prepolymer powder, that isnot susceptible to microwave heating, is heated solely by conduction,oven air convection and oven surface radiant heat at ambient pressure torelease a blowing agent in a continuous foaming process. In theproduction of a polyimide foam, the polyimide prepolymer powder, with upto 80% of the theoretical carbon dioxide already released, is theprecursor or feed stock for the process.

The continuous foaming apparatus 10 comprises: a conveyor belt 12; aconveyor belt preheating unit 14; a powder feed system 16; an oven 18; apost oven cooling unit 20; a trimming and slicing unit 22; a post curingoven 24; a second cooling unit 26; a crushing unit 28; a slicing station31; a cut off station 32; and a conveyor belt cleaning device 34. Inaddition, a monitoring system 36 can be utilized to control the process.

In the manufacture of the polyimide foam product 38, the polyimideprepolymer powder is deposited on the conveyor belt 12 which has beenpreheated to about 175 to 290 degrees Centigrade. The layer of powder 40is conveyed through the oven 18 on the conveyor belt where the heat fromthe preheated conveyor belt 12 and the oven causes the polyimideprepolymer powder to react, foam and form a bun 42. Upon exiting theoven the continuous foam bun 42 is cooled, trimmed, sliced and cut intodiscrete lengths or, in addition to these process steps, the bun is alsosubjected to post curing and/or crushing.

The conveyor belt 12 is made of a material having a high thermalconductivity and a relatively low heat capacity so that it can berapidly heated, will readily transfer its heat to the prepolymer powderand can be rapidly cooled after exiting the oven 18. It is preferred touse conveyor belts made of stainless steel or carbon steel. Thesematerials have the required thermal conductivity, emissivity and heatcapacity. In addition, these materials have a high tensile strength andcan be operated under tension to keep the conveyor belt 12 flat andtracking correctly. The conveyor belt 12 must be kept flat to produce anacceptably smooth surface on the underside of the foam bun 42 and thefinished foam product 38. In use, stainless and carbon steel conveyorbelts also resist elongation and damage through abrasion or otherfactors.

Depending on the thickness of the polyimide prepolymer powder layer 40and the thickness of the continuous foam bun 42 being produced, thepreheated steel conveyor belt 12 may not have sufficient heat capacityby itself to supply the heat required to react the lower portion of theprepolymer powder layer 40. The heat capacity of the conveyor belt 12can be increased by increasing the thickness of the conveyor belt.However, the thickness of the conveyor belt 12 can only be increased tothe point where it will still be flexible enough to bend and pass overthe pulleys 44 It is contemplated that belts between 1.0 and 1.2 mmthick will be preferred but that belts ranging from 0.4 to 1.8 mm couldperform satisfactorily. Accordingly, for certain product thicknesses, anadditional heating unit 46 may be required to supply additional heat tothe underside of the conveyor belt 12 as it passes through the oven 18.The heating unit 46 can be a commercially available infrared heater or acontact heater.

The conveyor belt 12 is driven by a conventional variable speed drivesystem. The speed of the conveyor belt can be controlled by themonitoring system 36 which will regulate the speed to cause the finishedfoam product to meet certain preselected physical properties.

The conveyor belt preheating unit 14 employs a conventional,commercially available, infrared radiation or convection heating system.The heating system should have sufficient capacity to raise thetemperature of the conveyor belt 12 from the ambient temperaturesurrounding the apparatus 10 to a temperature between 175 and 290degrees Centigrade at the maximum operating speed of the conveyor belt12.

After the conveyor belt 12 exits the conveyor belt preheating unit 14and before the conveyor belt enters the oven 18, the layer 40 ofprepolymer powder is laid down on the upper surface of the conveyor beltby the prepolymer powder feed system 16. The powder feed system 16 canbe a hopper trough 48 with a screw feed that extends across the width ofthe conveyor belt 12 and deposits the powder layer evenly to acontrolled thickness across the width of the conveyor belt 12. Theamount of powder deposited on the conveyor per unit of time is based onthe speed of the conveyor 12 and the desired thickness and density ofthe finished product 38.

The oven 18 is a conventional foaming oven (e.g. gas fired oven) usinginfrared radiation and convection heating. With a polyimide prepolymerpowder, the cellular structure is essentially developed in about 10minutes and further chemical reaction and stabilization takes placeafter about an additional 10 minutes. Accordingly, the length of theoven 18 should be sufficient to permit the prepolymer powder layer 40 tohave a residence time in the oven of from 10 to 30 minutes depending onthe operating speed of the conveyor belt 12.

Upon exiting the oven 18, the continuous polyimide foam bun 42 passesthrough the post oven cooling unit 20. As the conveyor belt 12 passesthrough the post oven cooling unit, air is blown on the upper surface ofthe foam bun 42 and on the underside of the conveyor belt 12 byconventional cooling fans. Since the steel conveyor belt is readilycooled, the cooling air blown against the underside of the conveyor belt12 rapidly cools down the conveyor belt and the lower surface of thecontinuous foam bun 42 as the cooling air blown against the uppersurface of the continuous foam bun 42 cools the upper surface of thefoam bun.

The upper surface and sides of the foam bun 42 must be cooledsufficiently (e.g. below 120 degrees Fahrenheit) to impart sufficientintegrity to the foam to permit the skin 30 on the upper surface andsides of the bun to be removed in the trimming operation. By removingthe skin 30 that forms on the bun 42 in the oven 18, a smooth finishedsurface is provided for the end product 38 and better heat transfer tothe interior of the bun 42 is achieved in the post curing oven 24 whenthe bun is being post cured. The skin 30 trimmed from the continuousfoam bun can be ground into particles by conventional grinding machinesand introduced into the prepolymer powder as a filler to reduceproduction costs.

As the continuous foam bun 42 exits the post oven cooling unit 20 whereit has been cooled, the upper surface and the sides of the foam bun 42are trimmed by hot wire cutters or ban saws of the trimming unit 22 toremove any rough portions or skin 30. The hot wire cutters or ban sawsof the trimming and slicing unit 22 are conventional and commerciallyavailable and provide the finished product 42 with smooth planarsurfaces. A hot wire cutter such as the Treffner HWC 20--Universal hotwire cutter manufactured by Treffner Engineering can be used to performthe trimming operation. After trimming, the continuous foam bun 42 canbe cut into discrete lengths and removed from the conveyor belt 12 asfinished product 38 or the foam bun 42 can be post cured in the postcuring oven 24. Upon exiting the oven 18, the ingredients of theprepolymer powder are not fully reacted. The post curing of the foam bun42 is employed to more fully complete the reaction. This results inimproved product performance by reducing smoke, weight loss anddimensional changes in the product when it is subjected to hightemperatures and it may also improve the dimensional stability of thefinished product due to a reduction in internal stresses. The postcuring oven 24 is heated by conventional infrared radiation and/orconvection heaters and operates at temperatures between about 200 and315 degrees Centigrade. When post curing is used in the process, the bun42 is treated in the post curing oven 24 for from 5 to 60 minutes andthe post curing oven should be sized accordingly.

When the bun 42 is post cured in the post curing oven 24, the bun iscooled for a second time in the second cooling unit 26. As with the postoven cooling unit 20, in the second cooling unit 26 air is blown on theupper surface of the foam bun 42 and on the underside of the conveyorbelt 12 by conventional cooling fans to cool the bun. The surfaces ofthe continuous bun 42 are cooled to 120 degrees Fahrenheit or less. Thisprepares the bun for the optional crushing step, the optional slicingstep and the cutting step where the bun is cut into discrete lengths.

The foam bun 42 is crushed by passing the bun through the crushing unit28. The crushing unit comprises two conventionally driven crushing rolls50 and 52 which have a surface speed equal to that of the bun 42. Whenthe crushing unit is not in use, the upper crushing roll 50 is raisedout of the way, e.g. by hydraulic cylinders, so that the bun 42 passesover the lower crushing roll 52 without engaging the upper crushingroll. When the crushing unit 28 is in use, the upper crushing roll 50 islowered until the peripheral surfaces of the upper and lower rolls 50and 52 are spaced apart about 85% to 20% of the uncrushed thickness ofthe bun 42. The spacing of the rolls 50 and 52 relative to each otherdepends on the degree of crushing desired. The crushing of the bun to athickness of 85% to 20% of its uncrushed thickness makes the foam moreflexible and breaks cell walls to improve acoustic absorption. After thecrushing operation, the continuous foam bun regains substantially thethickness the bun had entering the crushing operation.

After passing through the crushing unit 28, the continuous foam bun 42is sliced into thinner sheets by a hot wire cutter in the slicing unit31 when thinner sheets are desired and cut into discrete lengths to formthe finished product 38 with a hot wire cutter or with a ban saw 26 inthe cut off unit 32. A hot wire cutter, such as, the Treffner HWC 20-Universal hot wire cutter manufactured by Treffner Engineering can beused in the slicing unit 31. In the cut off unit 32, a hot wire cuttersuch as the Treffner HWC 44 Profile and Downcutter, manufactured byTreffner Engineering, offers the advantage of a smoother surface withlittle or no dust. However, a ban saw is capable of higher cuttingspeeds and while the surface is not as smooth it is sufficiently smoothfor many applications. Once the foam product 38 is cut into discretelengths, the pieces 38 of foam are removed from the conveyor belt 12 forstorage or further processing and the conveyor belt begins its returnrun to the upstream end of the apparatus 10.

As the conveyor belt 12 passes around the downstream pulley 44 arotatable driven steel bristled brush 54 of the cleaning unit 34 brushesagainst the surface of the conveyor belt 12 and removes any polyimidefoam adhering to the upper surface of the conveyor belt. This cleans theconveyor belt 12 and prepares the conveyor belt to receive a new layer40 of prepolymer powder after the belt passes through the preheatingunit 14. In addition, the polyimide foam removed from the conveyor beltcan be ground and mixed with the prepolymer powder being fed into thedistribution trough 48 and used as a filler to reduce raw materialcosts.

It is contemplated that conventional, commercially available monitoringunits 55 and 59 can be used to monitor the mass of the prepolymer powderlayer 40 entering the oven 18 and the density of the foam bun 42 exitingthe oven 18. FIG. 2 shows the monitoring unit 55 which comprises a betaemitter 56 and a beta detector 58 for monitoring the prepolymer powderlayer 40 entering the oven 18. As shown the beta emitter 56 is locatedabove the conveyor belt 12 and the beta detector 58 is located beneaththe conveyor belt and directly beneath the beta emitter 56. Thus, themass of the prepolymer powder layer 40 entering the oven 18 is measuredby passing the beta beam vertically through the powder layer from theemitter 56 to the detector 58. The loss of beta particles is directlyproportional to the mass between the emitter and the detector. Since theconveyor belt 12 would have a constant mass, variations in the loss ofbeta particles would be directly related to the mass of the prepolymerpowder layer 40 on the conveyor belt 12. The beta gages can bestationary or the gages can be traversed back and forth across the widthof the conveyor belt 12 by conventional traversing mechanisms to monitorthe entire width of the prepolymer powder layer 40.

FIG. 3 shows the monitoring unit 59 which comprises a pair of beta gageslocated at the exit of the oven 18 to measure the mass and consequentlythe density of the foam bun 42 exiting the oven 18. The beta emitter 60is located on one side of and above the upper surface of the conveyorbelt 12 and the beta detector 62 is located on the opposite side of theconveyor belt and directly opposite the beta emitter 60. Thus, the betabeam is directed horizontally across the width of the conveyor belt andthrough the width of the foam bun 42. As discussed above, the loss ofbeta particles is directly proportional to the mass between the betaemitter and the beta detector. Thus, for a given width of foam bun 42,the loss of beta particles would be directly proportional to the mass ordensity of the foam bun 42 and the density of the foam bun can bemonitored and compared to the mass of prepolymer powder layer 40 beingintroduced into the oven 18 to control the process. FIG. 4 shows anarrangement for monitoring the degree of cure of the foam bun 42 exitingthe oven 18. As the polypher cures, the molecular linking in the polymerchanges and the dielectric properties of the polymer change. Thus, bymeasuring the properties of microwaves reflected from the interior ofthe foam polymer bun 42, the degree of cure of the polymer comprisingthe bun 42 can be determined. In this arrangement, a dielectric meter 64is positioned above the conveyor belt 12 and emits microwave energy thatpenetrates the bun 42 and is reflected back to the meter 64 by the foambun where the characteristics of the reflected microwaves are measuredto determine the degree of cure of the polymer in the bun 42.

After appropriate calibration, the beta gages and dielectric meter canrelate prepolymer powder input quantities, conveyor belt speeds, andconveyor belt and oven temperatures to desired product densities forbuns 42 of various thicknesses. This information can be used as input toa conventional controller which would regulate conveyor belt speed, theamount of prepolymer powder loaded onto the conveyor belt 12, theconveyor belt preheat temperature, the oven temperature or a combinationof two or more of these variables to control the properties of thefinished foam product.

FIG. 5 shows a second apparatus for preheating the conveyor belt 12before the prepolymer powder layer 40 is deposited onto the conveyorbelt. Instead of having a separate conveyor belt preheating unit 14, theupstream end of the conveyor belt 12 is enclosed in the oven 18. Byenclosing the upstream end of the conveyor belt 12 in the oven, the oven18 provides the heat to preheat the conveyor belt. In this arrangement,the hopper trough 48 is insulated from the heat of the oven 18 by arefractory insulation enclosure 66 to prevent the prepolymer powder frombeing heated above a temperature of about 40 degrees Centigrade.

In the method of the present invention, prepolymer powder with up toabout 80% of the carbon dioxide reacted, is deposited on the conveyorbelt 12 from the hopper trough 48 to form a layer 40 of prepolymerpowder on the conveyor belt. The conveyor belt 12 has been preheated tobetween 175 and 290 degrees Centigrade and heat transfer to the lowerportion of the prepolymer powder layer 40 from the conveyor beltcommences as the conveyor belt 12 conveys the prepolymer powder layerinto the oven 18.

Within the oven 18, heat is transferred by infrared radiation andconvection to the upper portion of the prepolymer powder layer 40 asheat is transferred to the lower portion of the prepolymer powder layerby conduction from the conveyor belt. The main heat source for the lowerone half of the prepolymer powder layer 40 is the heat conducted fromthe conveyor belt 12 and the main heat source for the upper half of theprepolymer powder layer is by convection and radiation from the ovensurfaces. Since the developing cellular structures in the upper andlower portions of the prepolymer powder layer do not form insulatingbarriers to interfere with the main heat sources for the upper and lowerportions of the layer 40, the developed cellular structure of theoverall foam bun 42 throughout its thickness is uniform producing animproved product. Should the heat capacity of the preheated conveyorbelt 12 be insufficient to provide the required heat to react theingredients in the lower half of the powder layer additional heat can beprovided to the conveyor belt 12 by the auxiliary heating unit 46.

After the continuous foam bun 42 exits the oven 18, the bun is cooled byair in the post oven cooling unit 20. The bun is cooled sufficiently toallow the bun to be trimmed and sliced into sheets by the trimming andslicing unit 22 and, if desired, to be removed from the conveyor beltbefore the core of the bun 42 has cooled.

After trimming, the continuous bun 42 can be post cured in the postcuring oven 24 to improve its performance. As discussed above, the postcuring of the bun reduces smoke, weight loss and dimensional changes inthe bun when the product is subjected to high temperatures in use. Thepost cure takes place at temperatures of from 190 to 315 degreesCentigrade and lasts for from 3 to 60 minutes.

After the post cure, the foam bun 42 is again cooled in the secondcooling unit 26 by directing air onto the upper surface of the bun andthe underside of the conveyor belt 12. Once cooled, the bun 42 can thenbe subjected to the crushing step by crushing the bun between crushingrolls or cylinders 50 and 52 to improve the flexibility and acousticalproperties of the bun or the bun can be merely passed over roll 52 intothe slicing and cutting stations 31 and 32.

After the post curing and/or crushing of the bun 42, or if no postcuring or crushing of the bun is performed, after the bun has beencooled and trimmed upon exiting from the oven 18, the continuous bun 42can be sliced into thinner sheets in the slicing station 31 and cut intodiscrete pieces 38 in the cutting station 32 to form the finishedproduct.

After the conveyor belt 12 transfers the continuous bun 42 to thecrushing station 28, the conveyor belt 12 is cleaned by the brush 54 toremove any polyimide foam adhering to the conveyor belt. The particlesof polyimide foam removed from the conveyor belt are collected and, ifdesired, the particles are ground and mixed with the prepolymer powderas a filler to reduce raw material costs. The cleaned conveyor beltreturns to the preheating unit 14 or the upstream end of the oven 18 tobe heated and prepared to receive the precursor prepolymer powder layer40 from the powder feed system 16 and the formation of the foam bun 42continues.

The process control system 36, through the beta gages 56 and 58 whichdirect a beta beam vertically through the prepolymer powder layer 40 andthe beta gages 60 and 62 which direct a beta beam horizontally throughthe foam bun 42 in a direction across the width of the conveyor belt 12monitor the mass of the prepolymer powder layer 40 entering the oven 18and the density of the foam bun 42 exiting the oven 18. The dielectricmeter 64 measures the degree of cure of the bun 42 exiting the oven 18.The beta gages and dielectric meter relay this information to aconventional control system 70 which adjusts the amount of prepolymerpowder loaded onto the conveyor belt 12; the speed of the conveyor belt12; the temperature of the preheated conveyor belt 12; the temperatureof the oven 18; and/or the heat input of the auxiliary heater 46 to theconveyor belt 12 to control the density and degree of cure of the bun 42exiting the oven 18. As an alternative, if a less expensive controlsystem is desired, an operator can take readings directly off the betagages and dielectric meter and make the necessary process adjustments.

In describing the invention certain embodiments have been used toillustrate the invention and the practice thereof. However, theinvention is not limited to these specific embodiments as otherembodiments and modifications within the spirit of the invention willreadily occur to those skilled in the art on reading this application.The invention is not intended to be limited to the specific embodimentsdisclosed, but is to be limited only by the claims appended hereto.

What is claimed is:
 1. A method of continuously foaming a polyimideprepolymer powder comprising:continuously depositing a prepolymer powderin a continuous layer on a continuous metallic conveyor belt that hasbeen preheated to a temperature of at least 175 degrees Centigrade;moving the continuous prepolymer powder layer through a heated oven onthe continuous metallic conveyor belt; heating an upper portion of theprepolymer powder layer by oven air convection and infrared radiationand heating a lower portion of the prepolymer powder layer by heatconduction from the metallic conveyor belt to react and foam theprepolymer powder layer to form a continuous piece of foam bun.
 2. Themethod of claim 1 including: cooling the metallic conveyor belt uponexiting the oven to cool a bottom surface of the continuous foam bun andenable the removal of the foam bun from the conveyor belt before aninterior portion of the foam bun has cooled.
 3. The method of claim 1including: cooling the continuous foam bun upon exiting the ovensufficiently to permit a skin to be removed from an upper surface of thecontinuous foam bun; trimming the skin from the upper surface of thecontinuous foam bun; reheating the continuous foam bun by passing thecontinuous foam bun through a post curing oven; cooling the continuousfoam bun a second time; and cutting the continuous foam bun intodiscrete lengths to form a foam product.
 4. The method of claim 3including: crushing the foam bun after it has been cooled for the secondtime and prior to cutting the continuous foam bun into discrete lengths.5. The method of claim 3 including: grinding the skin removed from thefoam bun into particles and adding the particles to the prepolymerpowder.
 6. The method of claim 1 including: cleaning the conveyor beltafter it is no longer in contact with the foam bun and prior to thepreheating of the conveyor belt to remove foam from the conveyor beltthat has become adhered to the conveyor belt during the curing of thefoam bun in the oven.
 7. The method of claim 6 including: grinding thefoam removed from the conveyor belt into particles and adding theparticles to the prepolymer powder.
 8. The method of claim 1 including:cooling the continuous foam bun upon exiting the oven and slicing thecontinuous foam bun into sheets.
 9. The method of claim 1 including:measuring the mass of the prepolymer powder layer entering the oven andthe density of the continuous foam bun exiting the oven to control theprocess.
 10. The method of claim 1 including: measuring the degree ofcure of the continuous foam bun as the continuous foam bun exits theoven.
 11. The method of claim 1 wherein: the prepolymer powder is apolyimide prepolymer powder which is not susceptible to heating bymicrowave energy.